Hybrid cars and electric vehicles are rapidly developed recently. In this background, in the field of braking of automobiles, there is a rapid change from mechanical braking to electrical braking.
Generally, to brake an automobile, a battery is used power source for hydraulic valve driving to control braking. If the battery is disconnected or broken by some cause or other, electric power is not supplied from the battery, and the vehicle cannot be braked or controlled. In case of such emergency, a small battery is often prepared as auxiliary power source. The battery, if small in size, is heavy, and a power source backup device using capacitor instead of battery is proposed in patent document 1.
However, if a small battery or a capacitor is used as auxiliary power source, when the start switch is turned off, due to motors or lamps mounted on the vehicle or dark current consuming devices such as automatic unlocking device (key-less entry system), the electric power of the battery is consumed. As a result, the battery voltage is lowered, and various problems come out.    [Patent document 1] Unexamined Japanese Patent Publication No. H5-116571